Moves child to a new position in the list of this children.
The list contains widgets packed START as well as widgets packed END, in the order that these widgets were added to this .
A widget’s position in the this children list determines where the widget is packed into this. A child widget at some position in the list will be packed just after all other widgets of the same packing type that appear earlier in the list.
| this |
a Box |
| child |
the Widget to move |
| position |
the new position for |